When a Shopify refund syncs to Amazon, the app converts your Shopify refund reason into one of Amazon's allowed reasons via the Map refund reasons mapping. If a Shopify reason isn't in that mapping, the Default reason for refund is used as the fallback so the refund still goes through.

Steps

1. Go to Settings → Order Settings → Cancelation and Refund.
2. Make sure Refunded orders is on (this setting is dependent on it).
3. Find Default reason for refund and toggle it on.
4. Pick an Amazon refund reason from the Default refund reason dropdown — typically GeneralAdjustment, but pick whichever fits your most common refund case.
5. Save.